The present invention relates to a process for enhancing the electroplating of non-conductive surfaces, such as the through holes of a printed circuit board (PCB). In particular, the invention comprises an improved process for plating non-conductive surfaces without the need for electroless plating. The improved process is particularly suitable for plating surfaces, such as printed circuit boards, which comprise both non-conductive and conductive (metallic) surfaces.
Printed circuit boards are generally composed of a non-conductive layer, such as an epoxy resin/glass fiber mixture, which is positioned between copper or nickel plates or foils, or other conductive metal layers. There can also be a multiplicity of these alternating layers. Commonly, holes are drilled through the PCB to establish a connection between the conductive metal layers at specific points in the board. The holes are then metallized to form a connection between the conductive materials, usually by plating.
In order to achieve conductivity and a consistent, reliable bond between the electroplated metal (usually copper), and the through holes, the through holes are usually first provided with a layer of electroless copper in a process which requires several steps, including, desmear, pre-activation, activation with a palladium/tin activator, application of an accelerator, electroless copper deposition and one or more rinses, before electroplating could be effected. The need for application of electroless copper can be avoided, it has been found, by the deposition of carbon on the through holes or other non-conductive surfaces which are to be electroplated. In this way, the long process time, complex chemistry requiring constant monitoring, and sensitivity of electroless baths can be avoided. Moreover, the expensive waste treatment often required with electroless copper and palladium/tin activators can be eliminated.
However, the use of a carbon deposition process may have several drawbacks. After carbon deposition, it takes several minutes before the non-conductive surface being plated is completely covered by the electroplated metal. This is especially significant where the surface to be plated is a through hole. Electroplating after treatment with carbon begins adjacent to the outer conductive surfaces (i.e., the copper foil) of the PCB and extends inward towards the center of the hole. This occurs from both sides of the through hole and the plating meets in the center and completes the connection.
The first practical teaching of a carbon black deposition system, which permits the elimination of electroless copper deposition prior to electroplating, was by Minten and Pismennaya in U.S. Pat. No. 4,724,005, the disclosure of which is incorporated herein by reference in its entirety. A long line of patents have issued concerning improvements to or variations in the process described in U.S. Pat. No. 4,724,005 including U.S. Pat. No. 5,139,642 to Randolph et al., the teachings of which are incorporated herein by reference in their entirety.
Current carbon based plating processes require a microetch step after the carbon application in order to remove the carbon from the copper surfaces of printed circuit boards (eg. circuitry and inner layer connections) so as to insure good copper to copper adhesion with the subsequent plating. Typically substantial microetching is needed to reliably remove the carbon. Usually 40–60 micro inches of copper is removed in the micro etch. However, microetching frequently causes problems, particularly in plating in the area of the copper-dielectric interface. In particular, etching the copper frequently also strips the carbon coating from the dielectric area directly adjacent to the copper, thereby creating an insulating barrier for electrical continuity in the subsequent electroplating. This barrier then leads to poor plating and defects such as voids, knit lines, and plating folds. The prior art requirement for microetching prior to electroplating in carbon-based processes is detailed in U.S. Pat. No. 4,964,959 (Column 10, lines 5–60), the entire teachings of which are incorporated herein by reference. U.S. Pat. No. 4,964,959 describes the use of a microetch to “flake off” the carbon on the copper surfaces.
This invention proposes a modification to one of the standard process steps in the copper plating cycle, namely the neutralization step in the desmear cycle. The modified neutralization step proposed accomplishes both the required neutralization function as well as the application of a thin sacrificial coating to the copper surfaces prior to the application of carbon. The sacrificial coating allows the subsequent removal of carbon from the copper surfaces to be achieved more reliably with less microetching. Thus, an improvement is provided without unwanted expansion of the process cycle.